The field to which the present invention relates is that generally known as electrical discharge machining sometimes hereinafter referred to as EDM in which a material is removed from an electrically conductive workpiece by the action of electrical gap discharges occurring between a tool electrode and the workpiece. A dielectric coolant fluid is circulated and recirculated through the gap, usually under pressure, throughout the machining operation. An electrode or a workpiece servo feed system is used to provide relative movement and thus maintain an optimum gap spacing between the electrode and the workpiece as the workpiece material is being removed.
In standard polarity, a negative voltage is applied to the electrode. In reverse polarity, a negative voltage is applied to the workpiece. Because of the large amount of capacitance in the EDM gap circuit, the positive and negative polarity setups greatly differ. We may have a good waveform at negative polarity and a much worse waveform at positive polarity, or vice versa. Especially during the fine finish cut, as the on-time or off-time becomes shorter, the waveform becomes worse. In order to make the waveform symmetric for both polarities, I have found that it is desireable to incorporate a push-pull circuit with the main switching transistor connected to the gap.
It is also possible to control the operation and the changeover between roughing and finishing at the different polarities by means of a programmable computer, microprocessor or similar input device. One type of electrical discharge machining power supply system operable with inputs from a programmable computer is shown in Oliver A. Bell, Jr. U.S. Pat. No. 4,071,729 issued on Jan. 31, 1978 for "ADAPTIVE CONTROL SYSTEM AND METHOD FOR ELECTRICAL DISCHARGE MACHINING". This patent is of common ownership with the present application.